The present invention generally comprises a bouncing suppresses suspension control system which suppressive bouncing of the vehicle occuring when the vehicle passes over a bump, manhole projecting from the road surface, hole and so forth which causes temporary bounding and/or rebounding motion on the vehicle and which will be referred to hereafter as "bump". More specifically, the invention relates to a suspension control system which assures riding comfort by precisely performing bouncing suppressive suspension control to vary damping characteristics of front and rear suspension systems independently of each other.
Various uses of road preview sensors have been proposed and developed. Fro example, SAE Technical Paper Series Nos. 630750 and 800520, respectively published on October, 1968 and February, 1980 show road preview sensors for use in suspension systems for obtaining optimum riding comfort and drivability. In addition, Japanese Patent First Publication No. 57-172808, published on Oct. 23, 1982 discloses a vehicle height control system which includes a sensor which detects rough road conditions and adjusts the vehicle height level depending upon road surface conditions. A vehicle height a level sensor is employed in the disclosed vehicle height control system for monitoring the relative displacement between the vehicle body and wheel axle. The output of the vehicle level sensor is compared with a reference level, which serves as a rough road criterion, and adjusts the vehicle height according to the road surface conditions.
Japanese Patent First Publication No. 58-30542, published on Feb. 23, 1983, discloses a variable damping force shock absorber with damping characteristics varying in accordance with vehicle driving conditons. In the disclosed system, the magnitude of relative displacement between the vehicle body and wheel axle is measured and a vehicle height variation indicative signal is derived from the measured displacement and the instantaneous vehicle speed. The vehicle height variation indiciative signal value is compared with a reference value which serves as a staff suspension criterion for adjustment of the damping characteristics of the shock absorber in accordance therewith.
On the other hand, Published Japanese Patent Application (Tokkai) No. Sho. 56-42739 discloses a suspension control system which controls the damping force produced in response to rolling forces depending upon vehicle speed and steering adjustments. The damping force is so controlled as to be HIGH when the vehicle speed is higher than a set speed and the steering adjustments through greater than a predetermined angle are performed. In addition, the owner of the present application has already disclosed a suspension control system in Published Japanese Utility Model Application (Jikkai) No. Sho. 56-147107. In this system, the damping force produced by a suspension shock absorber is adjusted between HIGH and LOW levels depending upon the vehicle speed, the magnitude of steering adjustments and the angular velocity of steering adjustments.
In such prior art, road condition dependent suspension control has been performed according to the road surface condition indicative parameter. Such prior art suspension control systems achieve driving stability of the vehicle and riding comfort to some extent. However, in the prior it has been difficult to perform road condition dependent suspension control in a precise manner so as to adapt damping characteristics or stiffness of the suspension precisely depending upon the road surface condition.
A typical defect or drawback of the conventional systems is apparent when the vehicle passes over a bump, manhole projecting from the road surface or a hole formed in the road. In such case, it is required to harden or stiffen the suspension for maintain damping characteristics hard enough for driving stability. Such demand for hard suspension temporarily occurs when passing over the bump, manhole or hole on the road surface and thereafter terminates. Since the conventional suspension control system controls damping characteristics solely depending upon amplitude of unevenness of the vehicle, such bump, manhole, or hole on the road surface cannot be distinguished from undulations, gravel or a rough road.
Furthermore, since the conventional control system controls the suspension to harden the damping characteristics of the suspension with electronic or and mechanical lag time, the change of damping characteristics is delayed for a certain period. This lag time prevents the conventional suspension control systems for performing control precisely adapted to the road surface condition. In addition, when the road surface sensor is provided at a position corresponding to either the front or rear suspension, the lag will cause a substantial delay in the change of the damping characteristics to harden the suspension after passing over the bump, manhole or hole. On the other hand, when the road condition sensor is provided at the front end of the vehicle, hardening of the suspension occurs in advance of passing the bump, manhole or hole. This is still not satisfactory in controlling the suspension in a precise manner.